The long-term goal is to define the molecular mechanisms of thrombin (T) inhibition by the serpins, heparin cofactor II (HCII) and plasminogen activator inhibitor-1 (PAI-1), implicated in arterial thrombosis. Thrombin localized on fibrin (Fbn) and the glycosaminoglycans (GAGs) dermatan sulfate (DS)and heparin, reacts with HCII and platelet PAI-1, in GAG-accelerated mechanisms different from thrombin inhibition by antithrombin (AT), accelerated by high affinity heparin, present only in trace amounts. Unlike AT, HCII is a unique inhibitor of arterial thrombosis, as only HCII in the presence of DS is capable of inhibiting Fbn-bound thrombin. Thrombin exosites I and II are hypothesized to play different roles in these processes. Exosite I binds HCII and PAI-1 directly, whereas exosite II - heparin binding may modulate HCII and PAI-1 turnover. These steps are absent in the T - AT reaction. DS bound outside exosite II is hypothesized to act as template for inhibition by HCII of exosite ll-blocked thrombin, meizothrombin (MzT), and MzT(desFI). The identity of this site; the HCII and PAI-1 substrate pathways; the mechanisms of DS- selective inhibition of Fbn-bound thrombin by HCII, and of fibrinogen (Fbg) and Fbn regulation of thrombin inhibition by PAI-1 are all unknown. The studies will resolve these significant gaps, by using fluorescence equilibrium binding, steady-state and rapid kinetics with native thrombin, HCII and PAI-1, and specific loss- of-function mutants. They will test the hypotheses: that the exosite roles in the GAG-catalyzed thrombin inactivation mechanisms by HCII, PAI-1 and AT are distinctly different; that GAG binding outside exosite II on thrombin mediates inhibition by HCII; and that Fbg and Fbn regulate thrombin inhibition by these serpins differentially. Specific aims are: (1) To quantitate binding and chemical steps in the sequence of molecular events in the GAG-catalyzed thrombin inactivation and substrate pathways of HCII and PAI-1, compared to AT; (2) Tocharacterize the DS-binding site outside exosite II in thrombin and MzT, and its role in thrombin and MzT inhibition; and (3) To determine the contributions of Fbg and Fbn binding to exosite I and GAGs in thrombin protection from HCII, PAI-1, and AT. These mechanism-based studies are relevant to understanding the selective, localized regulation of thrombin activity by HCII and PAI-1, and serpin turnover, in arterial clots. They may facilitate development of novel anticoagulants based on HCII and DS specifically targeted to arterial thrombosis.